1. Field of the Invention
This invention relates to the treatment of inflammatory bowel disease. More specifically, it relates to methods and pharmaceutical compositions for the treatment and prophylaxis of IBD.
2. State of the Art
The term inflammatory bowel disease ("IBD") describes a group of chronic inflammatory disorders of unknown causes involving the gastrointestinal tract ("GI tract"). The prevalence of IBD in the US is estimated to be 200 per 100,000 population (approximately 500,000 people). Patients with IBD can be divided into two major groups, those with ulcerative colitis ("UC") and those with Crohn's disease ("CD").
In patients with UC, there is an inflammatory reaction primarily involving the colonic mucosa. The inflammation is typically uniform and continuous with no intervening areas of normal mucosa. Surface mucosal cells as well as crypt epithelium and submucosa are involved in an inflammatory reaction with neutrophil infiltration. Ultimately this situation typically progresses to epithelial damage with loss of epithelial cells resulting in multiple ulcerations, fibrosis, dysplasia and longitudinal retraction of the colon.
CD differs from UC in that the inflammation extends through all layers of the intestinal wall and involves mesentery as well as lymph nodes. CD may affect any part of the alimentary canal from mouth to anus. The disease is often discontinuous- severely diseased segments of bowel are separated from apparently disease-free areas. In CD the bowel wall thickens which can lead to obstructions. Fistulas and fissures are not uncommon.
Clinically, IBD is characterized by diverse manifestations often resulting in a chronic unpredictable course. Bloody diarrhea and abdominal pain are often accompanied by fever and weight loss. Anemia is not uncommon, as is severe fatigue. Joint manifestations ranging from arthralgia to acute arthritis as well as abnormalities in liver function are commonly associated with IBD. Patients with IBD have an increased risk of colon carcinomas compared to the general population. During acute "attacks" of IBD, work and other normal activity are usually impossible, and often a patient is hospitalized.
Although the cause of IBD remains unknown, several factors such as genetic, infectious and immunologic susceptibility have been implicated. IBD is much more common in Caucasian, especially those of Jewish descent. The chronic inflammatory nature of the condition has prompted an intense search for a possible infectious cause. Although agents have been found which stimulate acute inflammation, none has been found to cause the chronic inflammation associated with IBD. The hypothesis that IBD is an autoimmune disease is supported by the previously mentioned extraintestinal manifestation of IBD as joint arthritis, and the known positive response to IBD by treatment with therapeutic agents such as adrenal glucocorticoids, cyclosporine and azathioprine, which are known to suppress immune response. In addition, the GI tract, more than any other organ of the body, is continuously exposed to potential antigenic substances such as proteins from food, bacterial byproducts (LPS), etc.
Once the diagnosis has been made, typically by endoscopy, the goals of therapy are to induce and maintain a remission. The least toxic agents with which patients are typically treated are the aminosalicylates. Sulfasalazine (Azulfidine), typically administered four times a day, consists of an active molecule of aminosalicylate (5-ASA) which is linked by an azo bond to a sulfapyridine. Anaerobic bacteria in the colon split the azo bond to release active 5-ASA. However, at least 20% of patients cannot tolerate sulfapyridine because it is associated with significant side-effects such as reversible sperm abnormalities, dyspepsia or allergic reactions to the sulpha component. These side effects are reduced in patients taking olsalazine. Neither sulfasalazine nor olsalazine are effective for the treatment of small bowel inflammation. However, other formulations of 5-ASA have been developed which are released in the small intestine (e.g. mesalamine and asacol). Normally it takes 6-8 weeks for 5-ASA therapy to show full efficacy.
Patients who do not respond to 5-ASA therapy, or who have a more severe disease, are prescribed corticosteroids. However, this is a short term therapy and cannot be used as a maintenance therapy. Clinical remission is achieved with corticosteroids within 2-4 weeks, however the side effects are significant and include a Cushing goldface, facial hair, severe mood swings and sleeplessness. The response to sulfasalazine and 5-aminosalicylate preparations is poor in Crohn's disease, fair to mild in early ulcerative colitis and poor in severe ulcerative colitis. If these agents fail, powerful immunosuppressive agents such as cyclosporine, prednisone, 6-mercaptopurine or azathioprine (converted in the liver to 6-mercaptopurine) are tried. For Crohn's disease patients, the use of corticosteroids and other immunosuppressives must be carefully monitored because of the high risk of intra-abdominal sepsis originating in the fistulas and abscesses common in this disease. Approximately 25% of IBD patients will require surgery (colectomy) during the course of the disease.
Oxygen-derived free radicals such as HO.circle-solid., the superoxide anion and other reactive oxygen species such as HOCl, have emerged as a common pathway of tissue injury in a wide variety of diseases whose underlying cause is an inappropriately vigorous and sustained immune response (failure to control or down regulate response to the initial, appropriate stimulus). Examples of other disease, in addition to IBD and arthritis, where this mechanism seems to be the operative cause are ARDS, septic shock, asthma, diabetes, multiple sclerosis, uveitis, etc. Typically both a cytokine-mediated immune response and a nonspecific inflammatory cascade are involved in the primary inappropriate response with both responses mediated through active oxygen species (oxidative stress). The inappropriate secondary response, also mediated through oxidative stress) may involve tissue damaging oxidation by neutrophils and tissue macrophages.
Various approaches have been taken to suppress this inappropriate inflammatory response. The efficacy of small molecule inhibitors of the various leukotriene, PAF and cyclooxygenase pathways have shown only limited efficacy, perhaps because blocking only one of many pathways does not provide a sufficiently large decrease in overall oxidative stress. The other approach has been the use of antibodies or cloned receptor molecules which target specific proteins in the inflammatory cascade such as IL-1, IL-6 or TNF-.alpha.. However, this approach is practical only for acute conditions like septic shock or ARDS where IV administration and antibody formation against the therapeutic protein is less of a concern. For a chronic condition like IBD, an orally active small molecule that is fully active when dosed once-a-day seems to be the ideal candidate.
Another approach to mitigating the oxidative stress resulting from an inflammatory response is to employ nitrone related therapeutics (NRTs). The prototype NRT is .alpha.-phenyl-t-butyl nitrone (PBN) shown below. ##STR1##
NRTs represent a new category of therapeutics with the inherent capacity to overcome the shortcomings of other previously studied compounds. PBN is relieved to trap a free radical (R.circle-solid.) by adding R as shown below with formation of the following more unreactive nitroxyl free radical. ##STR2##
Nitrones were first used as analytical tools capable of reacting with highly reactive radicals to yield free radical adducts that are much less reactive. In many cases, the free radical/NRT adduct complex is stable enough to allow in vivo isolation and quantitation using electron spin resonance (ESR). The concept of using nitrones as therapeutics in neurodegenerative diseases resulted from the observations that NRTs such as PBN trap reactive oxygen species and/or secondary free radicals following ischemia. This may occur by converting highly reactive, toxic radicals into much less reactive, much less toxic products. Certain NRTs have been shown to protect experimental animals from ischemia/reperfusion injury (stroke). NRTs, administered chronically, reverse the age-associated increase in oxidatively damaged protein and the age-associated decrease in the activity of the oxidative-sensitive enzyme, glutamine synthetase, in the brain.
Accompanying the NRT-mediated changes in oxidized protein and glutamine synthetase activity is a significant improvement in the performance of animals in behavioral tests measuring short-term spatial memory. It has been shown that prototype NRTs mitigate the effects of this inflammatory cascade in a number of in vivo models. Of particular interest is the consistent and well documented protection shown by PBN against the lethality induced by LPS in various rodent models of septic shock. Remarkably, PBN has also been shown to increase the life span of senescence accelerated mice by one third, perhaps by mitigating free radical damage. PBN has also been shown to block inducible nitric oxide synthetase ("iNOS"), the enzyme responsible for producing large amounts of the highly damaging NO.circle-solid.. Thus, PBN can both trap HO.circle-solid. and suppress formation of NO.circle-solid., potentially neutralizing the effects of the two agents considered to be the most damaging to tissue.
When evaluating the prospects of using an antioxidant to successfully treat IBD, it is perhaps also useful to consider that the anti-oxidant defense of the human colon is relatively deficient compared to human liver (mucosal levels of SOD, catalase and GSH representing 8%, 4% and 40%, respectively of liver levels), thus leaving the colon particularly sensitive to oxidative stress.
A considerable number of chemical modifications have been made to increase NRTs suitability as therapeutic agents. The effects of intrinsic chemical reactivity and radical trapping ability have been examined by substituting the phenyl ring with electron donating or electron withdrawing substituents. More water soluble analogues were made by, for example, carboxylate or sodium sulfonate substitution of the phenyl ring. Lipophilic analogues were made with functional group substitutions on either the phenyl ring or the nitronyl nitrogen. The alkyl nitrogen substituent was varied through the standard straight chain and branched C.sub.3 -C.sub.5 substituents. Nitrone isosteres and related compounds were also targeted and examined for efficacy. This approach lead to various classes of compounds, such as substituted ureas, amides, thioamides, azoxy derivatives, sulphones, and hydroxamic acids.
In addition, some benzamide compounds substantially similar to these nitrones have recently been shown to have activity in the treatment of Parkinson's disease, HIV dementia, and related conditions.
As a final aspect of background, in evaluating the effectiveness of compounds in the treatment of IBD, an in vivo model based upon trinitrobenzene sulfonic acid ("TNBS") is used.
References relating to these subjects include:
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